It is reported on results of a European research project on the DOL effect of structural sized glulam in tension perpendicular to the grain with special respect to different climates and volumes. The extensive experimental and theoretical investigations were performed jointly at VTT and FMPA -Otto-Graf-Institute in Finland and Germany, respectively. The test scheme comprised 80 curved beams and 180 prismatic tension specimens built-up of Nordic spruce laminations of grades C35 and C40 from one selected stand. The long-term investigations with stepwise loading regime were done in constant, cyclic and natural sheltered outdoor climates with constant stress volumes of 0.01 to 0.24 m 3. As a major general result it can be stated that the findings from curved beams and tension specimens are well consistent. The results of ramp load reference and long term tests in constant climates validated the magnitudes of volume and pure time effect as specified in Eurocode 5. In variable climates a pronouncedly increased DOL effect was obtained which however is not bound to a specific material degradation but results from superimposed eigenstresses caused by the transient moisture gradients. Mean stress levels at failure normalized to ramp load strength in case of variable climates were in the range of 45%-66% with times to failure less than one month. The theoretical modeling of the long term damage forwarded that an effective stress concept based on transient Weibull stresses is a suitable approach to handle the non-linear superposition of applied loads and climate actions. The climate induced effective tension stress perpendicular to the grain can be in the range of 0.3 N/mm 2 in case of severe service class 2 conditions. Wider cross-sections are relatively less influenced by moisture variations. With respect to design two alternative strategies to deal with the variable climate aspect are sensitive: introduction of additive climate bound design stresses or, more globally, a strength modification factor for accumulated time of loading, kmod, of about 0.55 for all load-duration classes in service class 2.This research was performed in the flame of a European project supported by EU grant AIR2-CT94-1057 which is gratefully acknowledged. festigkeit von Brettschichtholz ill Bauteilabmessungen bei Zugbeanspruchung rechtwinklig zur Faserrichtung unter besonderer Berticksichtigung unterschiedlicher Klimata und Volumina. Die umfangreichen experimentellen und theoretischen Untersuchungen wurden gemeinsam bei VTT in Finnland und bei der FMPA -Otto-Graf-Institut durchgeffihrt. Das Versuchsprogram umfat]te 80 gekrtimmte Tr~iger und 180 primatische Zugprfifk6rper aus Fichtenlamellen nordeurop~iischer Provenienz der Festigkeitsklassen C35 und C40 yon einem ausgew/ihlten Standort. Die Langzeituntersuchungen mit stufenweiser Belastung wurden in konstantem, zyklischem und natiirlichem iiberdachtem Auflenklima mit konstant beanspruchten Volumina yon 0.01 bis 0.24 m 3 vorgenommen. Als ein wichtiges allgemeines Ergebnis ist hervorzuheben, daft di...
beamsThe objective of an AIR PROIECT (1994PROIECT ( -1997 was to establish a new scientific basis for the determination of the long term performance of reconstituted timber (glulam and LVL) in a variable environment with a duration of load of a few months. Three current problems of timber engineering were treated: delayed fracture in bending of straight or notched beams, delayed fracture of curved beams by tension perpendicular to grain, with the same methodology and with a sufficient number of tests to obtain a good statistical and phenomenological understanding of the phenomena.
DOL-Effekt (Duration Of Load) in Tri~gern verschiedener Form und AbmessungZiel dieses Projektes war das Erarbeiten einer neuen wissenschaftlichen Grundlage zum Bestimmen des Langzeitverhaltens van verleimten Werkstoffen (Glulam und LVL) in wechselnder Umgebung und unter Belastung van einigen Monaten. Drei aktuelle Probleme des Ingenieurholzbaus wurden behandelt: a) der verz6gerte Biegebruch in einteiligen und b) keilgezinkten geraden Tr~igern, sowie c) der verz6gerte Bruch gekriimmter Tr/iger unter Zugbelastung senkrecht zur Faser. AUe Proben wurden mit der gleichen Versuchsanordnung und in ausreichender Probenzahl geprfift, um gesicherte statistische Assagen und ein gutes Verst~indnis der Ph/inomene zu erarbeiten.
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